1. Field of the Invention
The present invention generally relates to the field of electric cables, i.e. cables for electric power transmission, in particular, alternate current (AC) power transmission, more particularly to submarine electric cables substantially intended to be deployed underwater.
2. Description of the Related Art
A typical submarine cable for AC power transmission in the medium voltage (MV) and high voltage (HV) ranges (less than approximately 50 KV for MV, higher than 50 KV for HV) comprises one or more conductors (one conductor for single-phase power transmission, three conductors for 3-phases power transmission; cables with more than one conductor are also referred to as “multi-core” cables). Each conductor is surrounded by a conductor screen, an insulation structure typically comprising an insulation layer and an insulation screen, a water barrier layer, a metallic screen or sheath, and optionally a plastic sheath. In multi-core constructions, the core conductors are bundled, with the hollow space being filled with fillers and surrounded by a bedding made of tapes or yarns and possibly by a plastic sheath. An armour of metal wires or tapes is usually circumferentially provided over the bedding. In some applications, the armour may be covered by a polymeric sheath, or by one or more layers of yarns. A thin polymeric sheath can also be applied on each individual armour wire.
The armour is a structural reinforcing part having the function of strengthening the mechanical characteristics and performance of the cable during handling and installation thereof, as well as to provide resistance against external damage. The use of metal in the armour is particularly advisable in submarine cables due to the compressive forces eventually exerted thereon, which may be a problem for non-metallic armours.
Typically, the armour is made of one or two layers of wires, round or flat in shape, made of steel with low to medium carbon content (for example ranging from less than 0.015% to up to 2%). Steel is generally used due to its low cost, availability of supply and good mechanical properties. Other materials used for the cable armour can be galvanized (e.g. zinc-coated) steel, copper, brass, bronze. Galvanized steel is preferably used when the armour wires are exposed to the environment without any polymeric sheath or yarn layer, to ensure better resistance to corrosion.
In use, submarine cables are generally installed under water, typically buried under the bottom ground, but portions thereof may be laid in different environment; this is, for example, the case of shore ends of submarine links, intermediate islands crossing, contiguous land portions, edge of canals and similar situations. One critical aspect of these environments is often a worse thermal characteristics and/or higher temperature with respect to the situation in the offshore main route.
An important parameter of an electric cable is the current rating, i.e. the amount of current that the cable can safely carry continuously or in accordance to a given load scenario. If the current rating is exceeded for a length of time, the increase in temperature caused by the generated heat may damage the conductor insulation and cause permanent deterioration of electrical or mechanical properties of the cable. The current rating of a cable is used to determine the proper cable core size for a given load, or current drain. Factors influencing the current rating of a cable are the cable core size, the operational system parameters of the electric power distribution circuit, the type of insulation and materials used for all cable components and the installation condition and thermal characteristics of the surrounding environment.
In an AC power cable, the magnetic field generated by the current flowing in the conductor/s induces losses in ferromagnetic materials, such as low to medium carbon-containing steel used as armour wires. As “ferromagnetic material” is meant a material having high magnetic permeability, i.e. a material capable of concentrating magnetic flux by a factor of more than 10. The magnetic hysteresis is the lagging of changes in the magnetization of a substance caused changes in the magnetic field as the magnetic field is varied. The magnetic domains of the ferromagnetic material rotate with the magnetic field in alternate current cable. This rotation of magnetic domains in the material causes friction and heat. The heat produced by this friction is called magnetic hysteresis loss. Such an induced heat, added to that produced by the conductor/s due to the current transport, can hinder the overall current carrying capacity of the cable, especially when the cable is deployed in environment with low or null heat dissipation capability.
The magnetic hysteresis losses can amount up to 20% or more of the overall loss suffered by an AC cable in operation, depending on the material and size of the armour.
Another phenomenon possibly affecting the current rating of a cable is that of the eddy currents. In an AC cable eddy currents are induced in conductive material, such as the metal of the cable armour. Eddy currents cause energy to be lost in form of heat that, as already said above in connection with magnetic hysteresis loss, can hinder the overall cable current carrying capacity.
The eddy current losses can amount at about 2% of the overall loss suffered by an AC cable in operation.
In the case of submarine cables, the above-mentioned problem is particularly important in cable sections laid in zones different from that of the underwater bottom ground, said zones being characterized, for example, by higher external temperature and/or soil thermal resistivity and/or deeper cable burial depth, these conditions affecting the ability of the cable to dissipate heat.
U.S. Pat. No. 4,644,097 relates to an armored submarine cable. In particular, the cable is provided with a core containing conductors and a layer of armoring disposed on the outside of and surrounding the core, the layer including at least one section of heavy armor including at least one layer of heavy metal wires having ends in order that the cable may withstand mechanical forces applied thereto, at least one section of lightweight armor having ends, and a transition region in which the armor section and the lightweight armor section are joined in a manner such that the stiffness and flexibility of the cable are controlled. In particular, the cable has a metal wire armor in the shallow water sections and light weight non-metallic armor in the deep water section. The transitions between the shallow and deep water sections of the cable are made so as to obtain a gradual and controlled change in the flexibility of the cable. The end portions of the armor wires and elements should preferably be treated with mechanical and/or chemical means so as to increase the surface areas before applying a synthetic jointing material. The jointing material which preferably could be an epoxy resin may be applied by pressure molding or by other means.
No mention is made about specific materials to be used as heavy armour section or lightweight armour section, and no hint is provided about the characteristics thereof. The lightweight section can be non-metallic; in such case U.S. Pat. No. 4,644,097 teaches to position such section in the deep water where the losses problem of the present invention not severely affect the current rating capability of the cable.
U.S. Pat. No. 6,567,591 relates to a submarine cable with a length of armouring that surrounds the cable core and has armouring wires which are replaced in at least some portions in the longitudinal direction of the armouring by filler strands manufactured of a material having a lighter and a lower tensile strength than the armouring. The armouring wires are composed, for example, of steel, special steel, especially stainless steel, or aluminium. Filler strands formed from plastic meet these requirements. These may be non-reinforced thermoplastics or reinforced plastics, especially fiber-reinforced plastics, for example glass fiber-reinforced plastics. Such filler strands are lighter than the armouring wires, so that the weight of the submarine cable can be reduced by adapting the armouring to the prevailing pressure conditions. The cable armour has armouring of differing load-bearing capacity along its length.
The cable has armouring sections differing from the mechanical point of view, but this document is silent about environmental conditions possibly haring the current carrying capability of the cable.
U.S. Pat. No. 3,925,598 discloses an armored submarine cable including a cable core centrally of the cable and a plurality of armor wires extending substantially longitudinally of the cable around the core and spaced apart therefrom. Each of the armor wires comprises a plurality of lengthwise successively aligned sections of anticorrosive metal wires and an electric insulation means between each adjacent two of said sections of the metal wires.
Each of the armour wire modified according to U.S. Pat. No. 3,925,598 still suffers from magnetic hysteresis and eddy current losses.
U.S. Pat. No. 6,747,213 relates to a power transport cable structurally reinforced by incorporating at least one reinforcing wire or armoring having one or more layers of wires. In particular, the cable has at least one reinforcing or armoring wire made of composite steel having a steel core of standard type, and covered in a layer of stainless steel.
The above described cable armour has no changing in the longitudinal direction. A corrosion problem is solved, but the magnetic hysteresis losses are still present, due to the presence of a steel core of the standard type. The overall cost of the cable is increased.
The Applicant observes that although the problem of avoiding a reduction in the electrical power transport capability of an electric cable due to heat generated by losses in the cable armour might be solved by increasing the size of the cable, or of portions thereof, particularly of those cable sections which, in use, lay in the above-mentioned unfavourable conditions, such a solution is not satisfactory since it implies heavier and more expensive cables in the first case, or the installation of transition joints between cable sections of different cable sizes in the latter case. Also, having a cable made up of distinct sections of different size is not desirable, because the cable continuity is impaired which is detrimental for the cable mechanical resistance and thus requires careful handling during laying operation.
Another possibility to reduce the losses in the cable armour could be using a different material for the armour, particularly using a non-ferromagnetic metal like copper, bronze, brass, or stainless steel. Nevertheless, the use of these materials for making the whole cable armour significantly increases the cost of the cable; in some instances, the quantity of these materials could be minimised by using plastic spacers among wires, in order to reduce the costs, but in this case the mechanical resistance/protection of the cable would be reduced.
The Applicant has tackled the problem of how to avoid that the current transport capability of an electric cable be hampered by losses in the cable armour in some specific sections of the cable system route.
The Applicant has observed that, from an overall system point of view, it is in general not necessary that the current rating of the cable is increased throughout the whole cable system route, being sufficient to achieve this only in particular sections along the cable route where different and more critical environmental and installation conditions are present and cannot be avoided, like for example, but not limited to, higher outside temperature and/or soil thermal resistivity and/or deeper cable burial depth, or installation of the cable within ducts, presence of air gaps, presence of heat sources in proximity of the cable, and any other cause that could reduce the current rating of the cable in specific sections along the cable route.
The Applicant has found a solution that is effective in overcoming the above-mentioned problem, in a way that neither excessively increases the costs of the electric cables nor makes the handling and installation operations of the cables more critical.